## Abstract

Motivated by the need for robust and fast distributed computation in highly dynamic Peer-to-Peer (P2P) networks, we study algorithms for the fundamental distributed agreement problem. P2P networks are highly dynamic networks that experience heavy node churn (i.e., nodes join and leave the network continuously over time). Our goal is to design fast algorithms (running in a small number of rounds) that guarantee, despite high node churn rate, that almost all nodes reach a stable agreement. Our main contributions are randomized distributed algorithms that guarantee stable almost-everywhere agreement with high probability even under high adversarial churn in a polylogarithmic number of rounds. In particular, we present the following results: 1. An O (log ^{2} n)-round (n is the stable network size) randomized algorithm that achieves almost-everywhere agreement with high probability under up to linear churn per round (i.e., en, for some small constant ε > 0), assuming that the churn is controlled by an oblivious adversary (that has complete knowledge and control of what nodes join and leave and at what time and has unlimited computational power, but is oblivious to the random choices made by the algorithm). 2. An O(log m log^{3} n)-round randomized algorithm that achieves almost-everywhere agreement with high probability under up to ε√n churn per round (for some small ε > 0), where m is the size of the input value domain, that works even under an adaptive adversary (that also knows the past random choices made by the algorithm). Our algorithms are the first-known, fully-distributed, agreement algorithms that work under highly dynamic settings (i.e., high churn rates per step). Furthermore, they are localized (i.e., do not require any global topological knowledge), simple, and easy to implement. These algorithms can serve as building blocks for implementing other non-trivial distributed computing tasks in dynamic P2P networks.

Original language | English (US) |
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Title of host publication | Proceedings of the 23rd Annual ACM-SIAM Symposium on Discrete Algorithms, SODA 2012 |

Publisher | Association for Computing Machinery |

Pages | 551-569 |

Number of pages | 19 |

ISBN (Print) | 9781611972108 |

DOIs | |

State | Published - 2012 |

Externally published | Yes |

Event | 23rd Annual ACM-SIAM Symposium on Discrete Algorithms, SODA 2012 - Kyoto, Japan Duration: Jan 17 2012 → Jan 19 2012 |

### Publication series

Name | Proceedings of the Annual ACM-SIAM Symposium on Discrete Algorithms |
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### Conference

Conference | 23rd Annual ACM-SIAM Symposium on Discrete Algorithms, SODA 2012 |
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Country/Territory | Japan |

City | Kyoto |

Period | 1/17/12 → 1/19/12 |

## Keywords

- Distributed algorithm
- Dynamic network
- Expander graph
- Peer-to-Peer network
- Randomized algorithm
- Stable agreement

## ASJC Scopus subject areas

- Software
- Mathematics(all)